1. Field of the Invention
The instant invention relates to techniques for joining ceramic-containing materials to metal-containing or ceramic-containing materials. More specifically, the instant invention relates to joining ceramic bodies or metal-ceramic composite bodies to other bodies by means of brazing. Of particular interest are techniques to braze siliconized or reaction-bonded composite bodies such as reaction-bonded silicon carbide to ceramic bodies such as aluminum nitride, or to bulk silicon, particularly in the form of wafers or sheets.
2. Discussion of Related Art
U.S. Pat. No. 4,602,731 to Dockus discloses a direct liquid phase bonding of ceramics or carbides to metals or other ceramics that provides joints of high strength at a low temperature, and thereby not affecting the ceramic microstructure or mechanical properties. The technique utilizes an aluminum-silicon brazing sheet or aluminum filler metal, and plating a thin film of nickel, nickel-lead alloy or cobalt-lead alloy onto the brazing sheet. The plated brazing sheet is then placed between the metal and ceramic bodies to be bonded. A single firing at relatively low temperature (about 1100° F.-1200° F.) in an inert atmosphere or vacuum is sufficient to carry out the bonding. During the heating operation, the aluminum-silicon alloy reacts with nickel to form a ternary eutectic. The reaction liberates heat, which aids in the formation of the eutectic and promotes additional reactivity at the metal-liquid-ceramic interface. A shear strength of about 4500 psi (31 MPa) is reported.
U.S. Pat. No. 5,965,193 to Ning, et al. discloses a process for applying a metal layer to a ceramic body. In particular, the ceramic member is contacted to a melt of aluminum or aluminum-alloy, and moved relative to the melt, thereby producing a clean interface between the melt and the ceramic member so that the ceramic member is wetted by the melt. Upon cooling, the melt remains solidified on the surface of the ceramic member. An application of this technique is in production of electronic circuits. Specifically, a predetermined circuit pattern may be etched into the aluminum surface.
U.S. Pat. No. 6,033,787 to Nagase et al. discloses a ceramic circuit board with a heat sink that exhibits long life under heat cycles, and has improved heat dissipation. The novel ceramic circuit board is produced by laminating and bonding aluminum plates onto both sides of a Si3N4, AlN, or Al2O3 ceramic substrate using aluminum-silicon-based “brazing solders”, and a heat sink formed of Al/SiC composite is laminated and bonded onto a surface of the first or second aluminum plate. The Al/SiC heat sink typically is also bonded to the aluminum plate with the brazing solder. In another embodiment, however, instead of using a brazing solder, the heat sink may be bonded to the rest of the structure by means of an aluminum foil featuring layers of aluminum alloy placed on both sides of the aluminum foil, the aluminum alloy having a lower melting point than the aluminum foil. Still further, nickel may be plated onto the bonding surfaces of the heat sink and the aluminum alloy layer, which enables the heat sink to be easily bonded to the aluminum foil.
One problem with employing nickel in conjunction with aluminum or aluminum-silicon-based brazing alloys is that of the reaction between the nickel and the aluminum or silicon to produce intermetallic compounds. Such compounds typically exhibit brittle behavior, at least at the temperatures of interest, e.g., about 0° C. to 100° C. The brittle nature of these intermetallics can result in weak metallurgical joints.
The Nagase technique nevertheless always requires that a metal plate, e.g., aluminum plate, be bonded to the ceramic substrate by means of the aluminum-silicon brazing solder. Further, in that embodiment in which the Al/SiC heat sink is bonded to an aluminum plate without the brazing solder, the aluminum foil had to be coated on both sides with the “aluminum-silicon melting point lowering layers.”
The instant invention addresses these deficiencies.
In recent years, reaction-bonded silicon carbide (“RBSC”) has become a candidate material for such applications as structural components of semiconductor fabrication equipment, wafer tables or chucks, for example. In these and in other applications in which this material increasingly is being used, there is a need to bond RBSC to other RBSC bodies. There is also a need to be able to bond RBSC to ceramic dielectrics such as aluminum nitride to provide for electrical insulation.
RBSC has also been suggested as a candidate mirror material. This material is difficult to polish to the exactness required of a mirror application because the SiC and the Si constituents have very different hardnesses and therefore are abraded at different rates. The SiC phase often ends up “standing proud” above the level of the Si areas. In contrast, Si can be polished readily to the required smoothness, but does not possess sufficient stiffness. Thus, unless they are made sufficiently thick, such mirrors could distort under their own weight. However, in an application such as a wafer table for a semiconductor lithography machine, or for a space-based mirror, the extra weight exacts a performance penalty. Thus, some workers have made the substrate or support for the mirror from RBSC, and then applied a silicon coating, such as by evaporation, to form the reflective surface. However, the physical deposition rates of the silicon are excruciatingly slow, and very little polishing can be done after the silicon surface is applied, lest the polishing wear through the thickness of the coating.
RBSC usually contains residual silicon, typically in interconnected form and dispersed throughout the silicon carbide phase. Another recent development has been the modification of this residual infiltrant phase to substitute one or more metals for some or all of the silicon, thereby imparting more metallic character to such modified or “hybrid” RBSC bodies, e.g., toughness. Like the Al/SiC composites of Nagase, the modified RBSC's of the instant invention contain SiC, Si and Al. In spite of these compositional similarities, a brazement could not be achieved by applying the braze directly to the modified RBSC material, at least not when the RBSC contains appreciable amounts of aluminum. In contrast, the Al/SiC of Nagase was readily brazeable. Clearly, these metal-ceramic composites are not as similar as a mere comparison of chemical constituents might tend to imply. Among the differences noted by the instant inventors, however, is that Nagase teaches a Si content in the metal matrix phase of his Al/SiC composite only up to about 20 wt %, whereas the Si content of the metal matrix of the instant RBSC materials typically is higher, e.g., about 40 wt % Si or more. The use of chloride or fluoride-based fluxes helps in the RBSC bonding somewhat, but the bonding was still somewhat unreliable in the RBSC version containing 40% Si in the infiltrant phase, and at best yielded a shear strength of only about 20 MPa. Besides, for many of the applications contemplated by the instant invention, such as semiconductor fabrication, these fluxes are considered contaminants and their use is discouraged if not prohibited outright.
What is needed is a technique for metallurgically bonding ceramic-containing materials such as RBSC to metal-containing or ceramic-containing materials at relatively low temperature, reliably, and with high-strength, without the possibility of intermetallic formation and without the need to use halogen-based fluxes. It is also an object of the present invention to attach a surface layer of silicon to a rigid substrate such that the bonded silicon layer can then be polished, perhaps also including grinding before polishing, to make an optical quality mirror. The instant invention provides this solution.